1. Field of the Invention
This invention relates generally to power generation systems and, more particularly, this invention relates to electrochemical cells and methods for improving performance therein.
2. Description of the Related Art
Electrochemical cells utilizing a reactive metal anode, a nonreactive, electrically conductive cathode and an aqueous electrolyte are well known. Such cells are described in detail in numerous patents and publications, including Rowley U.S. Pat. No. 3,791,871 (Feb. 12, 1974), Tsai et al U.S. Pat. No. 3,976,509 (Aug. 24, 1976), and Galbraith U.S. Pat. No. 4,528,248 (July 9, 1985), the respective disclosures of which are incorporated herein by reference.
The cell disclosed in Rowley U.S. Pat. No. 3,791,871 typifies prior electrochemical cells utilizing a reactive metal anode which is highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are in contact with an aqueous electrolyte during cell operation. In the cell of the Rowley patent, the anode comprises an elemental alkali metal such as sodium or lithium, and the electrolyte comprises an aqueous solution of sodium hydroxide or lithium hydroxide, respectively, produced by the electrochemical reaction of the anodic metal with water.
The anode of the Rowley patent is coated with a thin film of a nonreactive, partially water soluble material which is not electrically conductive. Preferably, the film is the natural hydrated oxide which forms on the metal surface as it is exposed to humid air. However, other suitable water soluble insulators may serve as the film. The film is porous and allows transport of aqueous electrolyte to the anode and transport of reaction products away from the anode.
A cathode comprising a nonreactive, electrically conductive metal is spaced from the anode to define a reaction zone therebetween.
The electrolyte of the cell disclosed in the Rowley patent is formed by the electrochemical reaction of water and the anodic metal. Thus, in the Rowley cell, water is introduced to the cell at a restricted rate and brought into direct contact with both the cathode and anode. The water dissolves a portion of the soluble film on the anode, resulting in the production of a hydrated hydroxide of the anode material, plus heat. As the reaction proceeds, useful electrical power is produced.
The anode and the cathode are not in direct contact with each other, but circuit connections are made at each of the cathode and anode for drawing electrical power from the cell.
The alkali metal of the anode is highly reactive with water. This reactivity decreases as the concentration of metal hydroxide in the electrolyte increases.
The electrolyte is preferably a hydroxide of the alkali metal utilized as the anode since such hydroxide is naturally formed during operation of the cell and hence automatically regenerates the electrolyte during operation.
Optimally (at typical operating temperatures), the concentration of lithium hydroxide in the electrolyte is maintained at about 4.2-4.5 molar. As the lithium hydroxide concentration in the cell rises, the rate of power generation from the cell correspondingly diminishes, and passivation of the anode can occur if the electrolyte becomes saturated with lithium hydroxide.
Thus, in these electrochemical cells, relatively high reactive metal hydroxide concentrations must be avoided to maintain a desired level of power output at operating temperatures.
Therefore, steps must be taken to maintain the reactive metal hydroxide concentration in the electrolyte at a level at which useful electrical current is produced.
Electrolyte management, i.e., the removal of the reactive metal hydroxide from the circulated electrolyte, presents a significant burden to reactive metal/aqueous electrolyte electrochemical cells.